1. Field of the Invention
The invention relates to a direct conversion receiver and a reception method in direct conversion receivers. The invention relates particularly to removal of IQ (In-phase/Quadrature) phase imbalance in direct conversion receivers.
2. Description of the Related Art
The use of digital wireless communication systems has recently been increasing. Systems of many different types have been introduced. For example, systems like Wireless LANs (Local Area Networks), UMTS (Universal Mobile Telecommunications System) and GSM (Global System for Mobile Communication) are gaining more attention and users are given more alternatives in wireless communication. To get customers interested in new services it is essential that the equipment needed in order to use the services should be priced correctly. Less expensive receivers having low power consumption are thus needed.
One solution to provide affordable receivers with low power consumption is to use a direct conversion analog front-end architecture in the receivers. In the direct conversion solution, a received RF signal is mixed directly into the base band and thereafter analog-to-digital converted. For the mixing process, two signals, a sine and a cosine signal, have to be provided. Because of technical reasons the precise orthogonality of both sinusoidal signals cannot be guaranteed; therefore an angle φ≠90° is measurable between the sine and cosine functions. This phenomenon is commonly called IQ phase imbalance.
Analog base band components, such as low-pass filters and base-band amplifiers are installed twice: one component for the I-branch and one component for the Q-branch. Because of manufacturing tolerances, or temperature influences, each component of a certain functional type may behave slightly differently compared with its counterpart on the other branch. The conjunction of frequency dependent base band devices with the constant IQ phase imbalance imperfections result in frequency selective IQ phase imbalance inaccuracies.
Because of the IQ imbalance, so-called image signals are created at image frequencies of the actual signals, causing IQ-cross talk. FIGS. 1A and 1B show an example of the reception of a multi-carrier signal under the effect of IQ imbalance. As shown in FIG. 1A, the received signal contains six carriers 100 to 114, out of which carriers 100 to 104 form the lower sideband and carriers 110 to 114 above the local oscillator (LO) frequency form the upper sideband. FIG. 1B shows how the signal spectrum of FIG. 1A may look after frequency conversion to base band when sufficient image rejection is not performed. In the situation shown by FIG. 1B, image signals 110A-114A overlap the converted signals 100A-104A.
For example, in case of two carriers with significant power difference in the received signal, an image of the first component appears in the second component and vice versa. The strengths of the image signals depend on the degree of the imbalance and on the strengths of the signals. Weak image signals are usually not a problem when the signal components have approximately the same magnitude. However, problems may arise when one signal is strong and the other signal is weak. If the image of the strong signal is stronger than the weak signal, or even comparable to it, the reception of the weak signal is degraded or even entirely prevented. In the example of FIG. 1B, image signal 110A of signal 110 is stronger than the weak signal 104A which may prevent reception of signal 104A.